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SubjectRe: [patch 3/3] ipc: increase IPCMNI_MAX
On Thu, 20 May 2010 17:07:41 +1000
Nick Piggin <npiggin@suse.de> wrote:

> Just wondering whether there is a good reason to have a full 16 bits of
> sequence in ipc ids? 32K indexes is pretty easy to overflow, if only in
> stress tests for now. I was doing some big aim7 stress testing, which
> required this patch, but it's not exactly a realistic workload :)
>
> But the sequence seems like it just helps slightly with buggy apps, and
> if the app is buggy then it can by definition mess up its own ids
> anyway? So I don't see that such amount of seq is required.
>
> Index: linux-2.6/ipc/util.h
> ===================================================================
> --- linux-2.6.orig/ipc/util.h
> +++ linux-2.6/ipc/util.h
> @@ -14,7 +14,16 @@
> #include <linux/err.h>
>
> /* IPCMNI_MAX should be <= MAX_INT, absolute limit for ipc arrays */
> -#define IPCMNI_MAX_SHIFT 15
> +/*
> + * IPC ids consist of an index into the idr, which allocates from the bottom
> + * up, and a sequence number which is continually incremented. Valid indexes
> + * are from 0..IPCMNI_MAX (or further constrained by sysctls or other limits).
> + * The sequence number prevents ids from being reused quickly. The sequence
> + * number resides in the top part of the 'int' after IPCMNI_MAX.
> + *
> + * Increasing IPCMNI_MAX reduces the sequence wrap interval.
> + */
> +#define IPCMNI_MAX_SHIFT 20
> #define IPCMNI_MAX (1 << IPCMNI_MAX_SHIFT)
>
> #define SEQ_SHIFT IPCMNI_MAX_SHIFT

Some anaylsis of the worst-case memory consumption would be mollifying.

I took the absence of Signed-off-by:'s to mean "rfc" and wandered away.


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